An image processing apparatus wherein a docu­ment on a document support table and an optical system are moved relative to each other for scanning to read the image of the document as a density signal, and the read signal is processed as required, the apparatus comprising a document mat so colored that when the document support table is covered with the mat, the read signal represents an intermediate color, discrim­inating means for judging whether the density signal represents the intermediate color to produce a binary signal, memory means having a memory capacity of 1/n based on the density signal, write control means for causing the binary signal to be written in the memory means in synchronism with the scanning movement, and means for determining the size of the document from the contents of the memory means.